1. Field of the Invention
The present invention relates to an interactive structure for gyrotrons, more particularly to a mode-selective interactive structure for gyrotrons.
2. Description of the Related Art
In order for a gyrotron to provide terahertz-wave radiation with super high output power, a high-order mode instead of a fundamental mode is used as an operating mode of the gyrotron. However, since the cutoff frequencies of adjacent high-order transverse modes are close, severe mode competition may hamper the performance of the gyrotron.
FIG. 1 is a frequency f to propagation constant kz diagram illustrating the competing modes that may be produced when tuning the operating frequency of a gyrotron, wherein curved lines represents different modes exist in the waveguide structure of the gyrotron, and sloped lines are the fundamental (s=1) and second (s=2) cyclotron harmonic beam-wave resonance lines. The oscillation occurs at where a mode-representing curved line intersects with a beam-wave resonance line. For example, suppose a high-order mode such as TE01 mode is the operating mode of the gyrotron, represented using a solid curved line, the oscillations occur at where the curved line representing the TE01 mode intersects with the s=1 beam-wave resonance line. However, the s=1 beam-wave resonance line also intersects with the curved line of other modes such as TE21 mode and TE31 mode; as a result, parasitic oscillations from TE21 mode and TE31 may occur within the operating region of the electron beam, a phenomenon known as mode competition. Besides, when the gyrotron changes the operating frequency by adjusting the magnetic field, the s=1 beam-wave resonance line is translated vertically and intersects with the curved line of TE01 mode at different frequencies, resulting in new competition modes such as TE41.
A prior art gyrotron disposes a groove on the wall of a circular waveguide or a resonance cavity so that when passing by the groove, a circular mode such as TE01, which has a wall surface current surrounding the central axis of the waveguide, is not affected, while a competing mode, which has a wall surface current in the axial direction, is substantially affected; hence, the propagation of the competing mode is hampered.
The prior art gyrotron has not arranged any lossy material or has arranged a low resistive loss material for the groove because the super high power absorbed may burn any lossy material. It relies on reflecting the competing modes by the groove to diverge the competing modes, but in such way, the competing modes may still exist and compete with the operating mode. Besides, the prior art gyrotron may need to shorten its interactive section in order to suppress the production of competing modes, and thus reduce the room for output power optimization.
In order to solve the aforementioned problems, the present invention is directed to providing a mode-selective interactive structure for gyrotrons which is capable of suppressing competing modes so that the operating mode may stand out from the mode competition thereby achieving mode selection.